If you haven't already brushed up on those concepts, just visit those articles hyperlinked above.

In the meantime, it's time to venture into the world of ISO and learn what it does and how it can impact the look and feel of your images.

What the Heck is ISO Anyway?

Back in the film days, ISO was often referred to as ASA, and it referred to the level of sensitivity of film to light.

The scale on which that sensitivity was measured was 100, 200, 400, and so on, with a lower number indicating a lesser sensitivity to light.

So, if you picked up a roll of ISO 100 film, you understood that due to its relative insensitivity to light that it would be most useful for photos taken in bright lighting conditions. You also understood that a roll of ISO 400 film was better suited for lower lighting conditions, like taking photos indoors.

When digital photography came about, the manner in which ISO was measured stayed the same.

That means that when you dial in an ISO setting on your camera, 100 is still less sensitive than 200 which is less sensitive than 400, and so on.

The biggest difference, however, is that where in the film days ISO referred to the physical qualities of the film you used, it now refers to an electronic process inside your camera.

Nevertheless, the concept is still the same - ISO controls the sensitivity of your camera's sensor to light.

For a great introduction to ISO, check out the video above by Sydney Portraits.

A Quick Scenario: Brightening Your Images

But let's say that you find that the images you're taking are just a little too dark and you want them brightened up.

That's where ISO comes in.

On many cameras, there's an ISO button on the top or the back of the camera body that allows you to quickly select a new ISO.

So, for example, if your indoor images are too dark when shooting at ISO 200, you can switch to ISO 400 to brighten the image.

This works because, with every movement of the ISO value upward, you double the sensitivity of the sensor to light. Of course, the inverse is true as well - with every movement of the ISO value downward, you halve the sensitivity of the sensor to light.

A Quick Scenario: Getting a Larger Depth of Field

If you're in aperture priority mode and you want a larger depth of field, ISO once again comes in handy.

For example, let's say you want to create an image like the one above in which everything from the foreground to the background is in sharp focus.

The problem is that in low lighting conditions, choosing a smaller aperture means you restrict the light entering the lens. Doing so in aperture priority mode means that the camera will select a shutter speed to maintain a good exposure.

So, if there's not a lot of light, that means the camera will select a very slow shutter speed which could end up being too slow to get a crisp, sharp image if you're holding the camera in your hand.

Again, ISO can come to the rescue.

If you can't get the aperture small enough for the desired depth of field without getting a shutter speed that's too slow, simply increase the ISO.

The increased sensitivity will compensate for the smaller aperture and allow you to shoot with a shutter speed that's fast enough for you to hold the camera as well.

A Quick Scenario: Getting a Faster Shutter Speed

Assume that you're at an indoor event photographing your child play a sport and that you're shooting in shutter priority mode.

Sports benefit from faster shutter speeds because you can freeze the movement of your subject, even if they are moving very quickly.

However, sometimes the lighting indoors isn't sufficient for a shutter speed that allows you to freeze movement.

Again, that's where ISO comes in.

Let's say you try taking a photo like the one above, but the movement of the girl's arm as she serves the ball is blurry with a shutter speed of 1/250th of a second.

To freeze that movement, you might need to dial in a shutter speed of 1/500th of a second.

The problem is that the change from 1/250th to 1/500th of a second halves the time the shutter is open. That means you need to compensate for that loss of light somehow.

You can do that by bumping up the ISO - if you're using ISO 400, changing it to ISO 800 would do the trick.

A Quick Scenario: Shooting in Full Auto Mode

When shooting in full auto mode, the camera makes all the decisions regarding exposure settings on your behalf.

How to Decide on an ISO Setting

To freeze movement, use a higher ISO setting so you can, in turn, use a faster shutter speed.

If you're shooting handheld, use a higher ISO setting to help prevent blurry photos, again, because you can use a faster shutter speed.

If you want a grainy look, use a higher ISO setting.

Conversely, if you have more than enough light - like shooting a portrait outdoors during the day - keep the ISO low so as to minimize digital noise. Likewise, if you're shooting with a tripod and need to brighten the image, you can use a slower shutter speed to get more light instead of using a higher ISO and dealing with noise.

See ISO in action and learn a little more about how ISO can impact your shots in the video above by Mike Browne.

Here's a final disclaimer: reading up on ISO is just the start! To really grasp how powerful of a tool it can be, you'll need to get out there and practice manipulating the ISO.

With practice, you'll develop a better sense of what ISO settings get you what results, and from there you can begin to manipulate ISO with more purpose and confidence.

Once you have the aperture, shutter speed, and ISO down pat, the next step is to learn how the three work together as part of the exposure triangle. Read more about that in this post.

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